Taking effect from 2020, the pact would target emissions from fossil fuels such as coal, oil and gas -- the backbone of the world's energy supply today -- and channel hundreds of billions of dollars in aid to vulnerable countries.

More than 150 world leaders launched the Paris talks Monday, seeking to build momentum for the tough negotiations ahead with lofty rhetoric about the urgency of the task.

But negotiators said the nitty-gritty discussions for a hugely complex 54-page draft text, riddled with undecided clauses, were advancing too slowly.

- 'Growing frustration' -

"We are not making anywhere near the progress we need to be making at this point," said Daniel Reifsnyder, one of the two co-chairmen in the talks' key arena.

Delegates, gathered at a highly-secured conference centre on the northern outskirts of Paris, remain deeply split over the key issues of finance for developing nations and burden-sharing, said a European negotiator who asked not to be named.

"There is a growing frustration," the European source said, with bureaucrats refusing to budge on the wording of certain sections of a draft text, but "some progress" being made elsewhere.

Such frustrations are typical of the start of climate negotiations, where vast interests are at stake and a single word in an agreement can have big repercussions, said veteran observers.

"I remain confident that it will be a hard fought two weeks but at the end of the day we are likely to achieve, and I believe we will achieve, an agreement," Australian Environment Minister Greg Hunt Hunt told reporters.

UN climate chief Christiana Figueres cautioned against despair.

"The text of the agreement will go through ups and downs, there will be many commas inserted and commas removed because that is the nature of this. It is a legally binding text and needs to be reviewed very, very carefully," she said.

Touching on the rich-poor issue, British charity Oxfam issued a study saying the wealthiest 10 percent of people produce half of Earth's climate-harming fossil-fuel emissions, while the poorest half contribute a mere 10 percent.

An average person among the richest one percent emits 175 times more carbon than his or her counterpart among the bottom 10 percent, the charity said.

Developing countries say the West has polluted for much longer and should shoulder a bigger obligation for cutting back.

- Finance wrangle -

They are also calling on rich nations to make good on a 2009 pledge to muster $100 billion (94 billion euros) a year in climate aid by 2020.

"The finance issue is the most difficult here," said Alden Meyer, of the respected US observer group the Union of Concerned Scientists (UCS). "People are not putting a lot of things on the table."

China briefly grabbed the limelight in Paris as its State Council vowed to slash emissions from coal-fired power stations by 60 percent by 2020.

The move would cut carbon dioxide emissions by 180 million tonnes annually, the official Xinhua news agency reported.

But specialists said Beijing's announcement, which coincided with growing discontent over choking pollution levels in China, offered nothing new.

"These are old numbers," said Lo Sze Ping, head of the World Wide Fund for Nature's China division. "These are the outcomes of what the government has committed to."

At the core of the talks is the goal of limiting warming to a maximum of two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

That objective -- along with a more ambitious option of 1.5 C -- has been enshrined in the UN Framework Convention on Climate Change (UNFCCC) since 2010.

Since then, scientists have pounded out an ever-louder warning that relentlessly climbing carbon emissions will doom future generations to rising seas and worsening floods, storms and drought -- a recipe for hunger, disease and homelessness for many millions.